authenticate via oauth2 and read a person record from the familysearch family tree api

Steps

1. Register a developer app at developers.familysearch.org to get a client_id; new keys are auto-enabled for the Integration (sandbox) environment only
2. For web/desktop/mobile apps, call the Authorization resource to redirect the user to the FamilySearch login page with your redirect_uri, receiving an authorization_code back
3. POST /cis-web/oauth2/v3/token with grant_type=authorization_code, the code, redirect_uri and client_id to get an access_token
4. For unattended/limited access, use grant_type=unauthenticated_session instead (only works against a small allow-list of resources like Places, Person Search, Date Authority)
5. Call GET /platform/tree/persons/{id} with Accept: application/x-gedcomx-v1+json and Authorization: Bearer {access_token}
6. Parse the returned gedcomx document's persons[] array for names, facts (type URIs like http://gedcomx.org/Birth), gender, living flag, and links to spouses/parents/children/sources